The present invention relates to an aluminum alloy on which an oxide film having a light gray is formed by anodization and to a method for producing the aluminum alloy.
In order to provide a decorative affect and improve corrosion resistance of a sheet material made of an aluminum alloy used for building materials, equipment, decorations, and others, an anodic oxide film is formed on the sheet material by anodic treatment. The anodization provides various colors dependent on types of alloys.
However, the anodic oxide film often takes on irregular tone. Furthermore, the tone of the color is liable to change with the lot of the alloy.
For example, on an aluminum alloy containing iron (Fe) and silicon (Si) as essential elements for coloring, an anodic oxide film having a gray based color is formed by an ordinary anodization. When the material of such an aluminum alloy is cast, iron and silicon are precipitated as intermetallic compound such as Al.sub.3 Fe, Al.sub.6 Fe, .beta.-AlFeSi, .alpha.-Al(FeM)Si, and where M are transition elements included in the aluminum alloy as impurities. Content ratios of these precipitations vary in accordance with compositions of the alloy, casting conditions, soaking treatment, and rolling process Sometimes, these precipitations are oxidized at anodic treatment or remain in the anodic oxide film without oxidized. The presence of the mixture of these precipitations cause irregular tone and the color instability in the anodic oxide film. For example, the tone of the color of the anodic oxide film delicately changes so that the anodic oxide film having a stable color can not be formed.
Japanese Patent Application Laid Open 60-82642 discloses a method in which an aluminum alloy ingot is heated at high temperature for a long time for transforming the Al-Fe system intermetallic compound to a stable Al.sub.3 Fe intermetallic compound in order to prevent the crystallization of some compounds which cause color instability.
Furthermore, there has been proposed a method in which an aluminum alloy ingot containing a large amount of iron is treated by soaking at low temperature so that Al.sub.6 Fe is prevented from transforming to Al.sub.3 Fe, and only the intermetallic compound mainly consisting of Al.sub.6 Fe is precipitated. Thus, a dark gray oxide film is formed on the aluminum alloy.
However, in the former process, the manufacturing cost increases and productivity is remarkably reduced because of the heat treatment at high temperature for a long period. The anodic oxide film does not take on gray, but takes on undesirable yellowish color. Since the color changes with the lot in dependence on a slight change of the conditions, it is necessary to strictly control the heating conditions of the ingot of the cast aluminum alloy.
In the latter process, the transformation of the Al-Fe system intermetallic compounds can be suppressed. However, the cast structure is not sufficiently homogenized at low temperature. Accordingly, the alloy having fine and uniform crystal structure is not obtained, and a stripe pattern tends to be formed on the anodized oxide film.